The field of the invention is power recovery from a fluid catalytic cracking (FCC) unit.
FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.
The heat of combustion in the regenerator typically produces flue gas at temperatures of 677° to 788° C. (1250° to 1450° F.) and at a pressure range of 138 to 276 kPa (20 to 40 psig). Although the pressure is relatively low, the extremely high temperature, high volume of flue gas from the regenerator contains sufficient kinetic energy to warrant economic recovery.
To recover energy from a flue gas stream, flue gas may be fed to a power recovery unit, which for example may include an expander turbine. The kinetic energy of the flue gas is transferred through blades of the expander to a rotor coupled either to a main air blower, to produce combustion air for the FCC regenerator, and/or to a generator to produce electrical power. Because of the pressure drop of 138 to 207 kPa (20 to 30 psi) across the expander turbine, the flue gas typically discharges with a temperature drop of approximately 125° to 167° C. (225 to 300° F.). The flue gas may be run to a steam generator for further energy recovery. A power recovery train may include several devices, such as an expander turbine, a generator, an air blower, a gear reducer, and a let-down steam turbine.
In order to reduce damage to components downstream of the regenerator, it is also known to remove flue gas solids. This is commonly accomplished with first and second stage separators, such as cyclones, located in the regenerator. Some systems also include a third stage separator (TSS) or even a fourth stage separator (FSS) to remove further fine particles, commonly referred to as “fines”.
The FCC process produces around 30% of the dry gas produced in a refinery. Dry gas comprises mainly methane, ethane and other light gases. Dry gas is separated from other FCC products at high pressures. FCC dry gas is heavily olefinic and typically used as fuel gas throughout a refinery. Olefinic dry gas, such as dry gas having over 10 wt-% olefins is not viable for use in gas turbines in which the olefins can cause internal fouling particularly due to the presence of diolefins. In some cases, FCC units produce more dry gas than the refinery consumes. The excess dry gas can be flared which is an environmental concern. To make less dry gas, the riser temperature can be reduced, adversely affecting the product slate, or throughput can be reduced, adversely affecting productivity. Olefinic dry gas can also be obtained from other unit operations such as those that are hydrogen deficient like cokers and steam crackers.